1. Field of the Invention
This invention relates to an optical disc recording method for recording an optical disc, and more particularly to an optical disc recording method wherein information is recorded onto an optical disc such as a magneto-optical disc when a movable portion of an optical head is positioned within a predetermined range.
2. Description of the Related Art
Information is recorded onto a recordable optical disc such as, for example, a magneto-optical disc, usually at the track pitch of about 1.6 .mu.m in its tracking direction in order to adjust a spot of a laser beam at a high speed to a track. Accordingly, an optical disc recording apparatus usually includes two actuators for rough adjustment and fine adjustment in order to cause a light beam from an optical head to trace any of tracks on which data are recorded at the track pitch of about 1.6 .mu.m. The actuator for fine adjustment is provided to cause a light beam to trace a track. The actuator for fine adjustment is provided usually on a movable portion of an optical head, but is sometimes provided separately from such movable portion. Meanwhile, the actuator for rough adjustment is provided to feed the optical head in a radiation direction of the disc.
A recording apparatus which employs a magneto-optical disc requires, in addition to an optical head, a magnetic head which generates a vertical magnetic field. In order to record information by a magnetic field modulating system, the magnetic head must generate a vertical magnetic field based on information to be recorded at an intensity sufficient to allow recording within the range in which the actuator for fine adjustment can possibly move. Accordingly, the magnetic head has a profile such that the length of a magnetic pole thereof in a radial direction of a magneto-optical disc is greater than that in a tangential direction of the magneto-optical disc.
Subsequently, an outline of a magneto-optical disc recording apparatus which uses a magneto-optical disc and the profile of a center magnetic pole of a magnetic head will be described.
FIG. 1 is a perspective view showing an exemplary one of a conventional magneto-optical disc recording apparatus. Referring to FIG. 1, the magneto-optical disc recording apparatus shown includes a laser diode 1 serving as a light source, a collimator lens 2 for converting a light beam emitted from the laser diode 1 into parallel light, and a beam splitter 3 of a trapezoidal shape. The beam splitter 3 has a first face 3a on which a dielectric multilayer film for separating a light beam emitted from the laser diode 1 and a return beam from the magneto-optical disc 6 from each other is provided, and a second face 3b on which a reflecting film for reflecting a return beam separated by and reflected from the first face 3a is provided. The magneto-optical disc recording apparatus further includes an objective lens driving apparatus 4 including two actuators for moving an objective lens 5 in a focusing direction and a tracking direction. The objective lens driving apparatus 4 includes a mirror for deflecting a light beam emitted from the laser diode 1 by 90 degrees toward the objective lens 5. The objective lens 5 is a single lens of the aspheric surface type. A magneto-optical disc 6 includes a substrate having a light transmitting property, a magneto-optical recording film provided on the substrate, and a protective film for protecting the magneto-optical disc film. The substrate of the magneto-optical disc 6 has pre-grooves formed at the track pitch of 1.6 .parallel.m in advance thereon for guiding a light beam. Recording of information onto the magneto-optical disc 6 is performed along the pre-grooves. The magneto-optical disc 6 is rotated at a fixed angular velocity by a spindle motor 9. A Wollaston prism 7 separates a return beam from the magneto-optical disc 6 into at least two light beams. A photo-detector 8 has a plurality of light receiving portions for receiving a plurality of light beams separated by the Wollaston prism 7. An optical head 12 includes the laser diode 1, collimator lens 2, beam splitter 3, objective lens driving apparatus 4, objective lens 5, Wollaston prism 7 and photo-detector 8 as described above.
The magneto-optical disc recording apparatus further includes a magnetic head 11 disposed in an opposing relationship to the objective lens driving apparatus 4 with the magneto-optical disc 6 positioned therebetween. The magnetic head 11 is connected to the objective lens driving apparatus 4 of the optical head 12 by way of a channel-shaped communicating portion 12a. The magnetic head 11 applies a vertical magnetic field, which is modulated in accordance with information to be recorded onto the magneto-optical disc 6 from the side of the magneto-optical recording film of the magneto-optical disc 6. An electromagnetic actuator of the objective lens driving apparatus 4 serves as an actuator for fine adjustment while a linear motor 10 serves as an actuator for rough adjustment.
The optical head 12 is moved in a radial direction of the magneto-optical disc 6 along the optical axis of a light beam emitted from the laser diode 1.
In FIG. 1, a light beam emitted from the laser diode 1 is converted into parallel light by the collimator lens 2. The parallel light advances toward the objective lens driving apparatus 4 by way of the first face 3a of the beam splitter 3. The light beam coming to the objective lens driving apparatus 4 is deflected by the mirror of the objective lens driving apparatus 4 and focused upon the recording film of the magneto-optical disc 6 by the objective lens 5. Meanwhile, a vertical magnetic field based on information to be recorded is applied from the magnetic head 11 to the magneto-optical disc 6. The recording film of the magneto-optical disc 6 is heated to a temperature higher than its Curie temperature by the light beam focused by the objective lens 5. As a result, the directivity of magnetization of the recording film of the magneto-optical disc 6 is erased, and, when the temperature of the recording film drops, the direction of magnetization of the recording film is determined by the vertical magnetic field which is applied thereto from the magnetic head 11 so as to record the information onto the magneto-optical disc 6.
The light beam is then reflected by the recording film of the magneto-optical disc 6 and advances to the beam splitter 3 by way of the objective lens 5 and the mirror of the objective lens driving apparatus 4. The light beam is thus reflected by the first face 3a of the beam splitter 3 and then reflected by the second face 3b of the beam splitter 3 so that it is introduced into the Wollaston prism 7. The light beam is then separated into at least two light beams by the Wollaston prism 7 and is then received by the photo-detector 8. A focusing error signal, a tracking error signal and other signals are thus formed based on an output signal of the photo-detector 8. The focusing error signal and the tracking error signal thus generated are supplied to the electromagnetic actuators of the objective lens driving apparatus 4 by way of servo circuits so that focusing and tracking are performed by the electromagnetic actuators of the object driving apparatus 4.
Meanwhile, a low frequency component of the tracking error signal thus generated is supplied to the linear motor 10 so that the optical head 12 is fed in a radial direction of the magneto-optical disc 6 in synchronism with recording onto the magneto-optical disc 6. Thus, the optical head 12 and the magnetic head 11 can be driven to the position of a desired track of the magneto-optical disc 6 by the linear motor 10 so as to record information on a desired location of the magneto-optical disc 6.
When information is recorded onto the magneto-optical disc 6, the spot of the light beam focused by the objective lens 5 on the magneto-optical disc 6 moves in a radial direction of the magneto-optical disc 6 within a certain range in a condition wherein the linear motor 10 remains stationary. Therefore, the magnetic head 11 must generate a magnetic field necessary for recording information within the range of movement of the spot on the magneto-optical disc 6.
The magnetic head 11 is constructed in the following manner.
FIGS. 2A and 2B are a plan view and a side elevational view, respectively, of a core of the magnetic head 11, and FIG. 2C is a side elevational view of the core of the magnetic head 11 on which a coil 26 is provided.
Referring to FIGS. 2A, 2B and 2C, the magnetic head 11 includes a base 21, and a disc-shaped core 23 provided on the base 21. The core 23 has a protrusion 22 formed at a central portion thereof, and has a pair of recesses 24 for leading out the opposite ends of the coil 26. A pair of U-shaped grooves 25 are formed on the opposite side faces of the core 23, and the coil 26 is secured to the base 21 by a protector 27 which is formed from, for example, Teflon. The coil 26 is wound around the protrusion 22 of the core 23. It is to be noted that reference numeral 28 denotes a track on the magneto-optical disc 6.
Magnetic fluxes are emitted radially from an end portion of the protrusion 22 of the core 23 of the magnetic head 11. As a result, the intensity of the magnetic field of the magnetic head 11 is substantially flat within the range of a central magnetic pole 29 corresponding to the protrusion 22 of the magnetic head 11 as shown in FIG. 3. In order to cover the range of movement of a beam spot on the magneto-optical disc 6, the length of the central magnetic pole 29 in a radial direction of the magneto-optical disc 6 must be made greater than the range of movement of the spot. In contrast, the length of the central magnetic pole 29 in a tangential direction of the magneto-optical disc 6 is sufficient if it covers the mounting error between the objective lens 5 and the magnetic head 11, for example, only .+-.50 .mu.m.
The central magnetic pole 29 of the magnetic head 11 in FIG. 3, or in other words, the protrusion 22 of the core 23, has a profile which is elongated in a radial direction of the magneto-optical disc 6 as seen from FIGS. 2A and 2B. Referring to FIGS. 2A and 2B, the protrusion 22 is 600 .mu.m in dimension in a radial direction of the magneto-optical disc 6 and 200 .mu.m in dimension in a tangential direction. The central magnetic pole 29 can cover the range of movement of the spot of .+-.300 .mu.m with respect to a track.
With the magneto-optical disc recording apparatus described above, however, the central magnetic pole 29, that is, the protrusion 22 of the core 23 of the magnetic head 11, increases in size as the range of movement of a beam spot increases. Consequently, also the diameter of the coil 26 increases, and the length of the wire material of the coil 26 increases. As a result, the inductance and the dc resistance of the magnetic head 11 increase and the power consumption of the magnetic head 11 increases, and also the amount of heat generated by the magnetic head 11 increases.
Generally, the magneto-optical disc 6 is accommodated in and used together with a disc cartridge. Thus, the magneto-optical disc recording apparatus includes a loading mechanism for a disc cartridge, and a mechanism for moving up or down the magnetic head in a timed relationship with a loading operation for a disc cartridge. As a result, a space necessary for a disc cartridge, the loading mechanism and a loading operation of the loading mechanism must be provided between the magneto-optical disc and the magnetic head. Consequently, a large gap is required between the magnetic head and the magnetic disc. If, under these conditions, an attempt is made to apply a magnetic field which is necessary for recording information onto the magneto-optical disc from the magnetic head, it is necessary to supply a high current to the magnetic head. Accordingly, there is a problem in that the power consumption and the amount of heat generated by the magnetic head are high.